HID: rmi: Use hid_report_len to compute the size of reports

Now that hid_report_len is in hid.h we can use this function instead of
duplicating the code which computes it.

Signed-off-by: Andrew Duggan <aduggan@synaptics.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>