mwifiex: Use macro MWIFIEX_MAX_BSS_NUM for specifying limit of interfaces

This macro is already used in mwifiex driver for specifying upper limit and
is defined to value 3. So use it also in struct ieee80211_iface_limit.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200626152938.12737-1-pali@kernel.org