TTY: msm_smd_tty: Fix driver addition/removal error

Currently, the functions smd_tty_add_driver and smd_tty_remove_driver
compare the channel name of the driver being added or removed to the
device name of already-registered drivers before completing the add
or remove. This is incorrect and causes driver removal to fail.

Compare the device name of the driver being added or removed instead
of the channel name in both functions (smd_tty_remove_driver and
smd_tty_add_driver).

Change-Id: I6a844428a57d0934ff9a4d7f7311cc933b2a1b80
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>