staging: fbtft: remove ARCH_BCM2708 optimization

ARCH_BCM2708 is not present in mainline so remove optimization.

Signed-off-by: Noralf Trønnes <notro@tronnes.org>
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>