brcmfmac: minor optimization of brcmf_sdbrcm_txpkt() function

When taking care of packet alignment to 64-byte boundary padding may
be added between SDPCM header and CDC data. It clear both SDPCM header
space and padding space. Changed it to only clear padding space. In
filling the SDPCM header it uses unaligned access to set SDPCM software
header, but preceding code assures it is properly aligned.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Change-Id: Iad22f277f3496440ba4d2db771205714774570ac
Reviewed-on: http://lb-bun-88.bun.broadcom.com:8080/76


Reviewed-by: Franky Lin <frankyl@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>