mwifiex: remove extra padding to AMSDU

Since commit: fb3c19bc, adding
extra padding to AMSDU is redundant since same is being performed at
mwifiex_11n_aggregate_pkt after forming the AMSDU packet. Fixing it
by removing it.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>