drivers:misc: ti-st: fix error codes

set-right the error codes that the shared transport driver
returns.
Instead of magic numbers like -1, return relevant codes such as
ETIMEDOUT or EIO, EAGAIN when wait times out or uart write bytes don't
match expected value or when registration fails and needs to be
attempted again.

Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>