mtd: char: Drop mtd_mutex usage from mtdchar_open()

It looks unnecessary in the function, remove it from the function
having in mind to remove it completely.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210217211845.43364-1-alexander.sverdlin@nokia.com