USB: storage: fix misplaced parenthesis

Due to a misplaced parenthesis the usbat_write_block() return value was not
stored, but a boolean. USB_STOR_TRANSPORT_NO_SENSE and USB_STOR_TRANSPORT_ERROR
were returned as USB_STOR_TRANSPORT_FAILED.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>