media: sp887x: fix a warning

drivers/media/dvb-frontends/sp887x.c:179 sp887x_initial_setup() error: memcpy() '&buf[2]' too small (30 vs 16384)

This is actually a false alarm, but reverting the check order
makes not only for humans to review the code, but also cleans
the warning.

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