rtw88: extract alloc rsvd_page and h2c skb routines

Extract skb allocation routines for rsvd_page and h2c.
These routines should also be used by USB and SDIO.

This should not change the logic at all.
memset() for pkt_info is unnecessary, just declare as {0}.
Also skb_put()/memcpy() can be replaced by skb_put_data().

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200312080852.16684-3-yhchuang@realtek.com