b43legacy: Fix sparse warnings

Sparse yields the following warnings for b43legacy:

  CHECK   drivers/net/wireless/b43legacy/phy.c
drivers/net/wireless/b43legacy/phy.c:1304:31: warning: potentially expensive pointer subtraction
drivers/net/wireless/b43legacy/phy.c:1304:31: warning: potentially expensive pointer subtraction
drivers/net/wireless/b43legacy/phy.c:1304:31: warning: potentially expensive pointer subtraction
  CHECK   drivers/net/wireless/b43legacy/debugfs.c
drivers/net/wireless/b43legacy/debugfs.c:243:9: warning: memset with byte count of 131072

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>