staging: wilc1000: linux_mon: fix coding style of kmalloc usage

This patch fixes coding style of kmalloc usage found by checkpatch.pl
CHECK: Prefer kmalloc(sizeof(*mgmt_tx)...) over kmalloc(sizeof(struct
tx_complete_mon_data)...)

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>