The platform_get_irq can return error. Assigning the return value to an
unsigned variable and checking it for negative value will always return
false.

Use an intermediate signed variable to get IRQ information, check for any
error and then assign it to 'irq' variable inside uart_port structure.
Signed-off-by: NKarthikeyan Ramasubramanian <kramasub@codeaurora.org>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: NStephen Boyd <swboyd@chromium.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>