brcmsmac: Remove the repeated declaration

Function 'brcms_c_stf_phy_txant_upd' are declared twice, remove
the repeated declaration.

Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1621844443-38290-1-git-send-email-zhangshaokun@hisilicon.com