OMAP/serial: Fix incorrect Rx FIFO threshold setting, LSR validation on Tx, and Tx FIFO IRQ generation

Original configuration of Rx FIFO threshold contained an error
that resulted Rx threshold to be effectively set to 1 character
instead of 16 characters, as noted in comments.
Checking LSR to contain UART_LSR_THRE bit set caused issue when
not all UART_IER_THRI interrupts have been properly handled.
This caused gap in Tx data, visible on high baud rates using
oscilloscope.
Setting OMAP_UART_SCR_TX_EMPTY bit in SCR caused UART_IER_THRI
interrupt to be raised only when Tx FIFO and Tx shift registers
are empty.
Signed-off-by: Alexey Pelykh <alexey.pelykh@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>