brcmfmac: fix txglomming scatter-gather packet transfers

The driver concatenates multiple packets in one MMC transfer. For
scatter-gather to work the total length need to be multiple of 512
bytes. A pre-allocated buffer was used to add padding to accomplish
that. However, the length was not properly set and it was freed after
the first transfer causing a crash.

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>