brcmfmac: remove unused variable data_len from brcmf_sdio_bus_txdata()

brcmfmac: remove unused variable data_len from brcmf_sdio_bus_txdata()

The local variable data_len is assigned but never used so get rid
of it.

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>