platform/x86: ideapad-laptop: Apply no_hw_rfkill to Y20-15IKBM, too

The commit 5d9f40b5 ("platform/x86: ideapad-laptop: Add
Y520-15IKBN to no_hw_rfkill") added the entry for Y20-15IKBN, and it
turned out that another variant, Y20-15IKBM, also requires the
no_hw_rfkill.

Trim the last letter from the string so that it matches to both
Y20-15IKBN and Y20-15IKBM models.

Bugzilla: https://bugzilla.opensuse.org/show_bug.cgi?id=1098626


Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>