staging: fbtft: fix sparse warning

fix the following sparse warning:
 drivers/staging/fbtft/fbtft-io.c:74:29: warning: incorrect type in assignment
 (different base types) drivers/staging/fbtft/fbtft-io.c:74:29:    expected
 unsigned long long [unsigned] [long] [long long] [usertype] <noident>
 drivers/staging/fbtft/fbtft-io.c:74:29:    got restricted __be64 [usertype]
 <noident>

Signed-off-by: Jandy Gou <gouqingsong@goodix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>