Input: synaptics-rmi4 - remove mutex calls while updating the firmware

This partially reverts commit 29fd0ec2 ("Input: synaptics-rmi4 -
add support for F34 device reflash")

irq_mutex should be used only to protect data->current_irq_mask, not
preventing incoming input to be processed while the upgrade of the
firmware is happening. We can simply disable the irqs when we don't
want them to interfere with the upgrade process.

Tested on S7300 and S7800 (with F34 v7 patch added)

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Nick Dyer <nick@shmanahar.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>