Fix below checkpatch issue found in mgmt_tx()

Prefer kmalloc(sizeof(*mgmt_tx)...) over kmalloc(sizeof(struct
p2p_mgmt_data)...)
Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>