b43: Fix assigning negative value to unsigned variable

fix warning reported by smatch:
drivers/net/wireless/broadcom/b43/phy_n.c:585 b43_nphy_adjust_lna_gain_table()
warn: assigning (-2) to unsigned variable '*(lna_gain[0])'

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/1648203315-28093-1-git-send-email-baihaowen@meizu.com