bcm2835gpio: Fix incorrect GPIO validation

bcm2835gpio: Fix incorrect GPIO validation

Incorrect validation prevented GPIO0 from controlling the direction of
the SWDIO buffer or operating TRST/SRST.

Have all GPIO number validation checks performed by is_gpio_valid().

Change-Id: Ib8fb704ab588a618ac41c111f6168d658891d92c
Signed-off-by: Steve Marple <stevemarple@googlemail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6938
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>