staging: rtlwifi: rtl8822b: fix to avoid potential NULL pointer dereference

skb allocated via dev_alloc_skb can fail and return a NULL pointer.
This patch avoids such a scenario and returns, consistent with other
invocations.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>