firmware: cleanup FIRMWARE_IN_KERNEL message

The help for FIRMWARE_IN_KERNEL still references the firmware_install
command that was recently removed by commit 5620a0d1 ("firmware:
delete in-kernel firmware").

Clean up the message to direct the user to their distribution's
linux-firmware package, and remove any reference to firmware being
included in the kernel source tree.

Fixes: 5620a0d1 ("firmware: delete in-kernel firmware").
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: stable <stable@vger.kernel.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>