Input: fix double equality sign in uevent

Looking at the uevent stream for input devices, all properties are on
the form "A=B" except the bitmap values, which are on the form
"A==B". This bug has been around at least since 2007, and the input
uevent code has been untouched since. The recent addition of device
properties suggests this is a good time for a remedy.

Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>