wifi: wcn36xx: Add debugfs entry to read firmware feature strings

Add in the ability to easily find the firmware feature bits reported in the
get feature exchange without having to compile-in debug prints.

root@linaro-alip:~# cat /sys/kernel/debug/ieee80211/phy0/wcn36xx/firmware_feat_caps
MCC
P2P
DOT11AC
SLM_SESSIONIZATION
DOT11AC_OPMODE
SAP32STA
TDLS
P2P_GO_NOA_DECOUPLE_INIT_SCAN
WLANACTIVE_OFFLOAD
BEACON_OFFLOAD
SCAN_OFFLOAD
BCN_MISS_OFFLOAD
STA_POWERSAVE
STA_ADVANCED_PWRSAVE
BCN_FILTER
RTT
RATECTRL
WOW
WLAN_ROAM_SCAN_OFFLOAD
SPECULATIVE_PS_POLL
IBSS_HEARTBEAT_OFFLOAD
WLAN_SCAN_OFFLOAD
WLAN_PERIODIC_TX_PTRN
ADVANCE_TDLS
BATCH_SCAN
FW_IN_TX_PATH
EXTENDED_NSOFFLOAD_SLOT
CH_SWITCH_V1
HT40_OBSS_SCAN
UPDATE_CHANNEL_LIST
WLAN_MCADDR_FLT
WLAN_CH144
TDLS_SCAN_COEXISTENCE
LINK_LAYER_STATS_MEAS
MU_MIMO
EXTENDED_SCAN
DYNAMIC_WMM_PS
MAC_SPOOFED_SCAN
FW_STATS
WPS_PRBRSP_TMPL
BCN_IE_FLT_DELTA

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220727161655.2286867-5-bryan.odonoghue@linaro.org