wil6210: allow empty WMI commands in debugfs wmi_send

There are many valid WMI commands with only header without any
additional payload. Such WMI commands could not be sent using
the debugfs wmi_send facility. Fix the code to allow sending
of such commands.

Change-Id: I581e099db5d2ee81be4345101aa54352b1d9564f
Signed-off-by: Lior David <qca_liord@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Git-commit: 69218a48
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git


CRs-Fixed: 1015627
Signed-off-by: Maya Erez <merez@codeaurora.org>