USB: serial: fix write memory-allocation flag

Fix regression introduced by commit 818f6036 ("USB: serial: add
memory flags to usb_serial_generic_write_start"), which incorrectly used
GFP_KERNEL in write(), which must not not sleep.

Reported-by: Dave Jones <davej@fedoraproject.org>
Tested-by: Dave Jones <davej@fedoraproject.org>
Cc: Dave Jones <davej@fedoraproject.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>