USB: s3c-hsotg: Fix interrupt cleaning code

This commit does the following:
1) clears all pending interrupts before unmasking;
2) clears interrupts as soon as possible to avoid missing
   next coming that may occur during handling;
3) removes ineffective interrupt cleaning code.

Signed-off-by: Anton Tikhomirov <av.tikhomirov@samsung.com>
Reviewed-by: Kyoungil Kim<ki0351.kim@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>