HID: intel-ish-hid: remove redundant variable num_frags

Variable num_frags is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable 'num_frags' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>