btmtk.ko is used by btusb.ko and btmtkuart.ko, add it into an own
package and make both packages depend on it.
Fixes: 1c42a0be3619 ("kernel: modules: bluetooth: separating UART and USB drivers")
Link: https://github.com/openwrt/openwrt/pull/18110
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
2389793edf