serial: qcom_geni_serial: Fix recent kdb hang

The commit e42d6c3e ("serial: qcom_geni_serial: Make kgdb work
even if UART isn't console") worked pretty well and I've been doing a
lot of debugging with it.  However, recently I typed "dmesg" in kdb
and then held the space key down to scroll through the pagination.  My
device hung.  This was repeatable and I found that it was introduced
with the aforementioned commit.

It turns out that there are some strange boundary cases in geni where
in some weird situations it will signal RX_LAST but then will put 0 in
RX_LAST_BYTE.  This means that the entire last FIFO entry is valid.
This weird corner case is handled in qcom_geni_serial_handle_rx()
where you can see that we only honor RX_LAST_BYTE if RX_LAST is set
_and_ RX_LAST_BYTE is non-zero.  If either of these is not true we use
BYTES_PER_FIFO_WORD (4) for the size of the last FIFO word.

Let's fix kgdb.  While at it, also use the proper #define for 4.

Fixes: e42d6c3e ("serial: qcom_geni_serial: Make kgdb work even if UART isn't console")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Akash Asthana <akashast@codeaurora.org>
Link: https://lore.kernel.org/r/20200806221904.1.I4455ff86f0ef5281c2a0cd0a4712db614548a5ca@changeidSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/qcom_geni_serial.c

@@ -361,11 +361,16 @@ static int qcom_geni_serial_get_char(struct uart_port *uport)
 			return NO_POLL_CHAR;
 
 		if (word_cnt == 1 && (status & RX_LAST))
+			/*
+			 * NOTE: If RX_LAST_BYTE_VALID is 0 it needs to be
+			 * treated as if it was BYTES_PER_FIFO_WORD.
+			 */
 			private_data->poll_cached_bytes_cnt =
 				(status & RX_LAST_BYTE_VALID_MSK) >>
 				RX_LAST_BYTE_VALID_SHFT;
-		else
-			private_data->poll_cached_bytes_cnt = 4;
+
+		if (private_data->poll_cached_bytes_cnt == 0)
+			private_data->poll_cached_bytes_cnt = BYTES_PER_FIFO_WORD;
 
 		private_data->poll_cached_bytes =
 			readl(uport->membase + SE_GENI_RX_FIFOn);