b43: silence most sparse warnings

  CHECK   drivers/net/wireless/b43/main.c
drivers/net/wireless/b43/main.c:111:5: warning: symbol 'b43_modparam_pio' was not declared. Should it be static?
  CHECK   drivers/net/wireless/b43/phy_g.c
drivers/net/wireless/b43/phy_g.c:975:56: warning: cast truncates bits from constant value (ffff7fff becomes 7fff)
  CHECK   drivers/net/wireless/b43/phy_lp.c
drivers/net/wireless/b43/phy_lp.c:2701:6: warning: symbol 'b43_lpphy_op_switch_analog' was not declared. Should it be static?
drivers/net/wireless/b43/phy_lp.c:1148:30: warning: cast truncates bits from constant value (ffff1fff becomes 1fff)
drivers/net/wireless/b43/phy_lp.c:1525:30: warning: cast truncates bits from constant value (ffff1fff becomes 1fff)
drivers/net/wireless/b43/phy_lp.c:1529:30: warning: cast truncates bits from constant value (ffff1fff becomes 1fff)
  CHECK   drivers/net/wireless/b43/wa.c
drivers/net/wireless/b43/wa.c:385:60: warning: cast truncates bits from constant value (ffff00ff becomes ff)
drivers/net/wireless/b43/wa.c:403:55: warning: cast truncates bits from constant value (ffff00ff becomes ff)
drivers/net/wireless/b43/wa.c:405:55: warning: cast truncates bits from constant value (ffff00ff becomes ff)
drivers/net/wireless/b43/wa.c:415:71: warning: cast truncates bits from constant value (ffff0fff becomes fff)

AFAICT, none of these amount to real bugs.  But this reduces warning
spam from sparse w/o significantly affecting readability of the code (IMHO).

Signed-off-by: John W. Linville <linville@tuxdriver.com>