at24: make input buffers of at24_*write() const

| drivers/misc/eeprom/at24.c:508: warning: assignment from incompatible pointer type

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>