pwm: ep93xx: Fix read of uninitialized variable ret

There is a potential path in function ep93xx_pwm_apply where ret is
never assigned a value and it is checked for an error code. Fix this
by ensuring ret is zero'd in the success path to avoid this issue.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: f6ef94edf0f6 ("pwm: ep93xx: Unfold legacy callbacks into ep93xx_pwm_apply()")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>