mtd: ubi: drop unexpected word 'a' in comments

there is an unexpected word 'a' in the comments that need to be dropped

file - drivers/mtd/ubi/vmt.c
line - 626,779

* Returns zero if volume is all right and a a negative error code if not.

changed to:

* Returns zero if volume is all right and a negative error code if not.

Signed-off-by: Jiang Jian <jiangjian@cdjrlc.com>
Signed-off-by: Richard Weinberger <richard@nod.at>