lightnvm: change max_phys_sect to uint

The max_phys_sect variable is defined as a char. We do a boundary check
to maximally allow 256 physical page descriptors per command. As we are
not indexing from zero. This expression is always false. Bump the
max_phys_sect to an unsigned int to support the range check.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jens Axboe <axboe@fb.com>