staging:iio:meter:ade7758: Fix timing on SPI read accessor functions.

According to the ADE7758 datasheet the minimum time between read command
(that is, a write to communication register) and data read must not be
less than 4us.

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>