staging: fbtft: propagate error code from kstrto*()

kstrto*() functions return proper error code.

Do propogate it to the user.

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>