serial: ifx6x60: add_timer is not safe in the mrdy_assert function

This patch make use of mod_timer instead of add_timer in the mrdy_assert function.
Because the srdy interrupter can go high when we are running function mrdy_assert and mrdy_assert
can be called by multi-entry. In our medfield platform, spi stress test can encounter this
error logs triggered by the BUG_ON of add_timer function.This patch had been tested on
our medfield platform.

the scenario:
      CPU0							CPU1
mrdy_assert
set_bit(IFX_SPI_STATE_TIMER_PENDING)
								ifx_spi_handle_srdy
								...
								clear_bit(IFX_SPI_STATE_TIMER_PENDING)
								...
								mrdy_assert
								set_bit(IFX_SPI_STATE_TIMER_PENDING)
								...
								add_timer
...
add_timer

Cc: liu chuansheng <chuansheng.liu@intel.com>
Cc: Bi Chao <chao.bi@intel.com>
Signed-off-by: Chen Jun <jun.d.chen@intel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/ifx6x60.c

@@ -231,9 +231,7 @@ static void mrdy_assert(struct ifx_spi_device *ifx_dev)
 	if (!val) {
 		if (!test_and_set_bit(IFX_SPI_STATE_TIMER_PENDING,
 				      &ifx_dev->flags)) {
-			ifx_dev->spi_timer.expires =
-				jiffies + IFX_SPI_TIMEOUT_SEC*HZ;
-			add_timer(&ifx_dev->spi_timer);
+			mod_timer(&ifx_dev->spi_timer,jiffies + IFX_SPI_TIMEOUT_SEC*HZ);
 
 		}
 	}