sparc32: use PUD rather than PGD to get PMD in srmmu_inherit_prom_mappings()

This is a misprint in the page table traversal in
srmmu_inherit_prom_mappings`() function which accessed a PMD entry using
PGD rather than PUD.

Since sparc32 has only 3 page table levels, the PGD and PUD are
essentially the same and usage of __nocache_fix() removed the type
checking.

Use PUD for the consistency and to avoid breakage because of upcoming
addition of type checking into __nocache_fix().

Fixes: 7235db26 ("sparc32: use pgtable-nopud instead of 4level-fixup")
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>