brcm80211: remove an unneeded NULL check

This code causes a static checker warning because "pi" gets dereferenced
before it is checked.  The dereference is inside the write_phy_reg()
function which is called from wlc_phy_write_txmacreg_nphy().

This code is only called from wlc_phy_init_nphy() and "pi" is a
valid pointer so we can remove the check for NULL.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>