mac80211: add dev_coredumpm() function

mac80211: add dev_coredumpm() function

dev_coredumpm() was added with kernel 4.7, but it is used by iwlwifi.
When the dev coredump framework form compat-wireless is used this is not
a problem because it already contains this, but this is deactivated if
the build system finds out that it is already included in the kernel we
compile against. This option was now activated by the bluetooth driver
btmrvl. Having dev coredump in the kernel adds about 400 bytes to the
lzma compressed kernel for brcm47xx.

This is copied from a more recent backports version to add the
dev_coredumpm() function when the internal core devdump is not used.

Fixes: a5922f6 ("kernel: bluetooth: add marvell sdio bluetooth module")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>