ARM: cleanup: regulator_get() error handling

regulator_get() does not return NULL as an error value.  Even when it
does return an error, the code as written falls out the error path
while returning zero (indicating no failure.)  Fix this, and use the
more correct IS_ERR() macro to check for errors.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>