USB: serial: mos7720: remove redundant variables iflag, mask and serial

Variables iflag, mask and serial are being assigned but are never used
hence are redundant and can be removed.

Cleans up clang warnings:
warning: variable 'iflag' set but not used [-Wunused-but-set-variable]
warning: variable 'mask' set but not used [-Wunused-but-set-variable]
warning: variable 'serial' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Johan Hovold <johan@kernel.org>