staging: fbtft: fix type assignment warning

Sparse spits out a warning that a __be16 was being assigned to a u16.
Change the type of txbuf16 to __be16 b/c it's a restricted type and
prevents mixing endianness.

Signed-off-by: Alex Wilson <alex.david.wilson@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>