usb: return 0 from usb_stor_get_info even if removable media

usb: return 0 from usb_stor_get_info even if removable media

This fixes a regression caused by

commit 07b2b78ce4bc8ae25e066c65245eaf58c0d9a67c
    dm: usb: Convert USB storage to use driver-model for block devs

which caused part_init to be called when it was not previously.
Without this patch, the following happens when a USB sd card reader is used.

=> usb start
starting USB...
USB0:   Port not available.
USB1:   USB EHCI 1.00
scanning bus 1 for devices... 3 USB Device(s) found
       scanning usb for storage devices... Device NOT ready
   Request Sense returned 02 3A 00
 ### ERROR ### Please RESET the board ###

This happens because dev_desc->blksz is 0.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>