staging: fbtft: differentiate between buffer and data types to fix sparse warning

sparse was complaning about an incorrect type cast:
drivers/staging/fbtft/fbtft-bus.c:60:1: warning: incorrect type in assignment (different base types)
drivers/staging/fbtft/fbtft-bus.c:60:1:    expected unsigned short [unsigned] [short] [usertype] <noident>
drivers/staging/fbtft/fbtft-bus.c:60:1:    got restricted __be16 [usertype] <noident>
drivers/staging/fbtft/fbtft-bus.c:60:1: warning: incorrect type in assignment (different base types)
drivers/staging/fbtft/fbtft-bus.c:60:1:    expected unsigned short [unsigned] [short] [usertype] <noident>
drivers/staging/fbtft/fbtft-bus.c:60:1:    got restricted __be16 [usertype] <noident>

The solution is to add an extra parameter to the macro to
differentiate between buffer type and data type.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Alfonso Lima Astor <alfonsolimaastor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>