wifi: brcmsmac: clean up unnecessary current_ampdu_cnt and related checks

In 'brcms_c_ffpld_check_txfunfl()', 'current_ampdu_cnt' is hardcoded to
zero, so 'txunfl_ratio' is always zero as well and some dead code can be
removed. Compile tested only.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Dmitry Kandybka <d.kandybka@gmail.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20240801101531.6626-1-d.kandybka@gmail.com