mtd: pmcmsp-flash: Allocating too much in init_msp_flash()

There is a cut and paste issue here.  The bug is that we are allocating
more memory than necessary for msp_maps.  We should be allocating enough
space for a map_info struct (144 bytes) but we instead allocate enough
for an mtd_info struct (1840 bytes).  It's a small waste.

The other part of this is not harmful but when we allocated msp_flash
then we allocated enough space fro a map_info pointer instead of an
mtd_info pointer.  But since pointers are the same size it works out
fine.

Anyway, I decided to clean up all three allocations a bit to make them
a bit more consistent and clear.

Fixes: 68aa0fa8 ('[MTD] PMC MSP71xx flash/rootfs mappings')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>