lightnvm: pblk: fix incorrect min_write_pgs

The calculation of pblk->min_write_pgs should only use the optimal
write size attribute provided by the drive, it does not correlate to
the memory page size of the system, which can be smaller or larger
than the LBA size reported.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Reviewed-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>