docs: gpio: driver.rst: fix a bad tag

With ReST, [foo]_ means a reference to foo, causing this warning:

    Documentation/driver-api/gpio/driver.rst:419: WARNING: Unknown target name: "devm".

Fix it by using a literal for the name.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/driver.rst

@@ -418,7 +418,7 @@ symbol:
 
 If there is a need to exclude certain GPIO lines from the IRQ domain handled by
 these helpers, we can set .irq.need_valid_mask of the gpiochip before
-[devm_]gpiochip_add_data() is called. This allocates an .irq.valid_mask with as
+``[devm_]gpiochip_add_data()`` is called. This allocates an .irq.valid_mask with as
 many bits set as there are GPIO lines in the chip, each bit representing line
 0..n-1. Drivers can exclude GPIO lines by clearing bits from this mask. The mask
 must be filled in before gpiochip_irqchip_add() or gpiochip_irqchip_add_nested()