wil6210: fix length check in __wmi_send

The current length check:
sizeof(cmd) + len > r->entry_size
will allow very large values of len (> U16_MAX - sizeof(cmd))
and can cause a buffer overflow. Fix the check to cover this case.
In addition, ensure the mailbox entry_size is not too small,
since this can also bypass the above check.

Change-Id: Iecb4f53ef05da0e015bc954b57b0e40debb7c8b7
Signed-off-by: Lior David <liord@codeaurora.org>