[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.