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>