[media] cxd2841er: don't use variable length arrays

The Linux stack is short; we need to be able to count the number
of bytes used at stack on each function. So, we don't like to
use variable-length arrays, as complained by smatch:

	drivers/media/dvb-frontends/cxd2841er.c:205:19: warning: Variable length array is used.

The max usecase of the driver seems to be 15 bytes + 1 for the
register.

So, let's be safe and allocate 17 bytes for the write buffer.
This should be enough to cover all cases. If not, let's print
an error message.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>