media: pt1: fix logic when pt1_nr_tables is zero or negative

pt1_nr_tables is a modprobe parameter. The way the logic
handles it, it can't be negative. However, user can
set it to zero.

If set to zero, however, it will cause troubles at
pt1_init_tables(), as reported by smatch:
	drivers/media/pci/pt1/pt1.c:468 pt1_init_tables() error: uninitialized symbol 'first_pfn'.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>