brcmfmac: use pre-allocated scatter-gather table for txglomming

brcmfmac: use pre-allocated scatter-gather table for txglomming

Instead of allocating a scatter-gather table for every transmit
reuse a pre-allocated table. The transmit path will be faster by
taking out this allocation.

Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>