mtd: concat: refactor concat_lock/concat_unlock

concat_lock() and concat_unlock() only differed in terms of the mtd_xx
operation they called. Refactor them to use a common helper function and
pass a boolean flag to indicate whether lock or unlock is needed.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Richard Weinberger <richard@nod.at>