HID: make hid_report_len as a static inline function in hid.h

In several hid drivers it is necessary to calculate the length of an
hid_report. This patch exports the existing static function hid_report_len of
hid-core.c as an inline function in hid.h

Signed-off-by: Mathieu Magnaudet <mathieu.magnaudet@enac.fr>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>