mtd: sa1100: avoid VLA in sa1100_setup_mtd

mtd: sa1100: avoid VLA in sa1100_setup_mtd

Enabling -Wvla found another variable-length array with randconfig
testing:

drivers/mtd/maps/sa1100-flash.c: In function 'sa1100_setup_mtd':
drivers/mtd/maps/sa1100-flash.c:224:10: error: ISO C90 forbids variable length array 'cdev' [-Werror=vla]

Dynamically allocate the cdev array passed to mtd_concat_create()
instead of using a VLA.

Reported-by: Arnd Bergmann <[email protected]>
Signed-off-by: Boris Brezillon <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Olof Johansson <[email protected]>