generic: platform/mikrotik: release mtd device after use

generic: platform/mikrotik: release mtd device after use

The code uses get_mtd_device_nm() which must be followed by a call to
put_mtd_device() once the handle is no longer used.

This fixes spurious shutdown console messages such as:
[   83.099037] Removing MTD device #1 (hard_config) with use count 1

Reported-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
[Backported from master]
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>