staging:iio:spear_adc: Fix sparse warning

The driver is casting from one __iomem pointer to another. Make sure to include
__iomem in the cast, otherwise sparse will complain with the following warning:

	drivers/staging/iio/adc/spear_adc.c:321:18: warning: cast removes address space of expression
	drivers/staging/iio/adc/spear_adc.c:320:33: warning: incorrect type in assignment (different address spaces)
	drivers/staging/iio/adc/spear_adc.c:320:33:    expected struct adc_regs_spear3xx [noderef] <asn:2>*adc_base_spear3xx
	drivers/staging/iio/adc/spear_adc.c:320:33:    got struct adc_regs_spear3xx *<noident>

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>