mtd: pmcmsp-flash: fix error paths in init_msp_flash

Cleanin up after errors in init_msp_flash().

Also cleanup_msp_flash() attempts to determine the size of
msp_flash with `sizeof(msp_flash) / sizeof(struct mtd_info **)'
This will not work since msp_flash is not an array.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>