staging: brcm80211: remove macro usage for sk_buff allocation

PKTGET macro calls osl_pktget. This function has been renamed to
pkt_buf_get_skb as it comprises of functionality additional to
dev_alloc_skb function and to get rid of the OSL concept in this driver.

Reviewed-by: Brett Rudley <brudley@broadcom.com>
Reviewed-by: Dowan Kim <dowan@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>