USB: mos7840: test and propagate set_uart_reg return value

The test for an mos7840_set_uart_reg() error return value only works when
status is signed. propagate its error value.

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Cc: SL Baur <steve@xemacs.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>