Input: synaptics-rmi4 - remove redundant null check on rmi_dev

rmi_dev is currently being dereferenced before it null checked, however,
after deeper inspecting, rmi_dev can never be null, so just remove this
redundant check.

Fixes CoverityScan CID 1391218 ("Dereference before null check")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>