media: pt1: fix strncmp() size warning

As warned by smatch:
	drivers/media/pci/pt1/pt1.c:213 config_demod() error: strncmp() '"tc90522sat"' too small (11 vs 20)

Use the same strncmp() syntax as pt1_init_frontends() does.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>