staging: fbtft: change 'gamma' array to u32

Having a local variable of 1024 bytes on 64-bit architectures is a bit
too much, and I ran into this warning while trying to see what functions
use the largest stack:

drivers/staging/fbtft/fbtft-sysfs.c: In function 'store_gamma_curve':
drivers/staging/fbtft/fbtft-sysfs.c:132:1: warning: the frame size of 1032 bytes is larger than 1024 bytes [-Wframe-larger-than=]

As there is no need for 64-bit gamma values (on 32-bit architectures,
we don't use those either), I'm changing the type from 'unsigned long'
to 'u32' here, which cuts the required space in half everywhere.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>