mtd: m25p80: fix allocation size

This patch fixes two memory errors:

1. During a probe failure (in mtd_device_parse_register?) the command
   buffer would not be freed.

2. The command buffer's size is determined based on the 'fast_read'
   boolean, but the assignment of fast_read is made after this
   allocation. Thus, the buffer may be allocated "too small".

To fix the first, just switch to the devres version of kzalloc.

To fix the second, increase MAX_CMD_SIZE unconditionally. It's not worth
saving a byte to fiddle around with the conditions here.

This problem was reported by Yuhang Wang a while back.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reported-by: Yuhang Wang <wangyuhang2014@gmail.com>
Reviewed-by: Sourav Poddar <sourav.poddar@ti.com>
Cc: <stable@vger.kernel.org>