[media] horus3a: 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/horus3a.c:57:19: warning: Variable length array is used.

The max usecase of the driver seems to be 5 bytes + 1 for the
register.

So, let's be safe and allocate 6 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>