tidspbridge: fix last patch to map same region of physical memory as before

Commit 559c71fe5dc3 ("Staging: TIDSPBRIDGE: Use vm_iomap_memory for
mmap-ing instead of remap_pfn_range") had the effect of inadvertently
shifting the start of the physical memory area mapped by
pdata->phys_mempool_base.  Correct this by subtracting that shift before
calling vm_iomap_memory() and adding it back afterwards.

Reported-by: Dheeraj CVR <cvr.dheeraj@gmail.com>
Signed-off-by: Steven Luo <steven@steven676.net>