b43: silence phy_n sparse warnings

drivers/net/wireless/b43/phy_n.c:512:53: warning: cast truncates bits from constant value (ffff0fff becomes fff)
drivers/net/wireless/b43/phy_n.c:765:66: warning: cast truncates bits from constant value (ffff7fff becomes 7fff)
drivers/net/wireless/b43/phy_n.c:1012:38: warning: cast truncates bits from constant value (ffff00ff becomes ff)
drivers/net/wireless/b43/phy_n.c:1119:38: warning: cast truncates bits from constant value (ffff0fff becomes fff)
drivers/net/wireless/b43/phy_n.c:2458:56: warning: cast truncates bits from constant value (ffff7fff becomes 7fff)
drivers/net/wireless/b43/phy_n.c:2933:38: warning: cast truncates bits from constant value (ffff0fff becomes fff)
drivers/net/wireless/b43/phy_n.c:3294:57: warning: cast truncates bits from constant value (ffff3fff becomes 3fff)

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