serial: omap: drop unnecessary check from remove

if platform_get_drvdata() returns NULL, that's
quite a nasty bug on the driver which we want to
catch ASAP. Otherwise, that check is hugely
unneeded.

Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>