carl9170: replace bitmap_zalloc with devm_bitmap_zalloc

the mem_bitmap is kept around for the lifetime of the
driver device. This is a perfect candidate for devm.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/0a79221aa5477501c3def032e95ef8018973a514.1646250537.git.chunkeey@gmail.com