iio: introduce type casts to avoid __ucmpdi2 calls

This patch type casts the switch control variable to 32 bits in order to
prevent a call __ucmpdi2 generated by some versions of gcc.

This fixes an undefined reference to `__ucmpdi2' when compiled for arch/blackfin

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>