iio: accel: mma9553: comment and error message fixes

Use "GPIO" instead of "gpio" and "ACPI" instead of "acpi".

Includes a couple of small style fixes in comments
(missing full stop, whitespace, paranthesis).

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Suggested-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>