mac80211: rt2x00: fix build warnings on 6.12 kernel

mac80211: rt2x00: fix build warnings on 6.12 kernel

Move rt2x00lib_read_eeprom() function prototype from rt2800lib.h to
rt2x00.h and make rt6352_enable_pa_pin() static to fix the missing
prototypes warnings:

/workspaces/openwrt/build_dir/target-x86_64_musl/linux-x86_64/mac80211-regular/backports-6.12.6/drivers/net/wireless/ralink/rt2x00/rt2x00eeprom.c:213:5: error: no previous prototype for 'rt2x00lib_read_eeprom' [-Werror=missing-prototypes]
  213 | int rt2x00lib_read_eeprom(struct rt2x00_dev *rt2x00dev)
      |     ^~~~~~~~~~~~~~~~~~~~~
/workspaces/openwrt/build_dir/target-x86_64_musl/linux-x86_64/mac80211-regular/backports-6.12.6/drivers/net/wireless/ralink/rt2x00/rt2800lib.c:308:6: error: no previous prototype for 'rt6352_enable_pa_pin' [-Werror=missing-prototypes]
  308 | void rt6352_enable_pa_pin(struct rt2x00_dev *rt2x00dev, int enable)
      |      ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18637
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>