wifi: ath9k: Don't mark channelmap stack variable read-only in ath9k_mci_update_wlan_channels()

This partially reverts commit e161d4b6.

Turns out the channelmap variable is not actually read-only, it's modified
through the MCI_GPM_CLR_CHANNEL_BIT() macro further down in the function,
so making it read-only causes page faults when that code is hit.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217183
Fixes: e161d4b6

 ("wifi: ath9k: Make arrays prof_prio and channelmap static const")
Cc: stable@vger.kernel.org
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230413214118.153781-1-toke@toke.dk