TTY: msm_smd_tty: Add warning to driver removal function

The current implementation of smd_tty_remove_driver does not check to
see if a matching driver instance is found before attempting to
unregister the driver. This could lead to a silent failure or an attempt
to remove a driver that does not exist.

Add a flag that indicates if a matching driver is found in the list
before removal is attempted. If no drivers are found, an error message
is logged.

Change-Id: I30f067b11dbe736ae065c5d4fbe30ca5c0645bb6
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>