[MTD] Make init_tqm_mtd function static

The init_tqm_mtd function does not need to be defined globally
in drivers/mtd/maps/tqm8xxl.c. Make it static.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>