Staging: iio: iio-trig-gpio: Remove redundant gpio_request

Remove redundant gpio_request:
The GPIO used as trigger IRQ, is also requested as gpio, but actually never read.

Use platform resource facility to get IRQs numbers and flags.
Make sure this driver can be used with any system IRQ, not necessarily limited to GPIO-IRQs.
Use dev_err(dev...) and friends instead of printk(KERN_ERR...)

Signed-off-by: Michael Hennerich <Michael.Hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>