Staging: fbtft: fix header guard typo

drivers/staging/fbtft/internal.h header guard tests for
__LINUX_FBTFT__INTERNAL_H but then defines __LINUX_FBTFT_INTERNAL_H
(only 1 underscore) and uses the same name for the #endif comment.
Use the same name everywhere.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>