usb: core: Copy parameter string correctly and remove superfluous null check

strsep() slices string, so the string gets copied by
param_set_copystring() at the end of quirks_param_set() is not the
original value.
Fix that by calling param_set_copystring() earlier.

The null check for val is unnecessary, the caller of quirks_param_set()
does not pass null string.
Remove the superfluous null check. This is found by Smatch.

Fixes: 027bd6ca ("usb: core: Add "quirks" parameter for usbcore")
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>