mwifiex: remove get operation code for SUBSCRIBE EVENT command

We don't send this command with GET action. Redundant code in
mwifiex_ret_subsc_evt() is removed in this patch.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>