usb: storage: use usb_store_dbg instead of US_DEBUGPX

The US_DEBUGPX macro uses printk without specifying a kernel log level, so
the default kernel log level is used, which may not match LOGLEVEL_DEBUG
used in usb_stor_dbg. Remove the macro and use usb_store_dbg instead.

Signed-off-by: Victor Dodon <printesoi@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>