wil6210: remove leftover "FIXME"s

"FIXME: IRQ mask debug" and "FIXME: interrupts enabled - for debug"
can be removed because wil6210_debug_irq_mask() is now considered
production feature.

"FIXME FW can transmit only ucast frames to peer" and "FIXME real
ring_id instead of hard coded 0" can be removed because FW/HW already
support multicast transmission.

Signed-off-by: Dedy Lansky <qca_dlansky@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>