usb: rockchip: on K_FW_LBA_WRITE_10 remove magic block size of 512 bytes

As well as in K_FW_LBA_READ_10 and K_FW_LBA_ERASE_10 take device's
block size from f_rkusb->desc->blksz instead of the fixed 512 bytes.

Keep original behaviour of retry probing assigned block device on
every host request to manage late SDCard plugs.

Signed-off-by: Alberto Panizzo <alberto@amarulasolutions.com>