wifi: b43legacy: remove unused freq_r3A_value function

clang with W=1 reports
drivers/net/wireless/broadcom/b43legacy/radio.c:1713:5: error:
  unused function 'freq_r3A_value' [-Werror,-Wunused-function]
u16 freq_r3A_value(u16 frequency)
    ^
This function is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230324135022.2649735-1-trix@redhat.com