usb: ums: fix disk capacity miscalculation and code cleanup

This patch prevents:
- ums disk capacity miscalculation because of integer overflow

Changes v2:
- Prevents passing zero size disk capacity to ums gadget driver
- Change function ums_get_capacity() to ums_disk_init() and do ums disk
  initialization before gadget init
- Remove unnecessary code from mass storage driver

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Marek Vasut <marex@denx.de>
diff --git a/include/usb_mass_storage.h b/include/usb_mass_storage.h
index 674ca70..9df3adc 100644
--- a/include/usb_mass_storage.h
+++ b/include/usb_mass_storage.h
@@ -25,12 +25,10 @@
 			   ulong start, lbaint_t blkcnt, void *buf);
 	int (*write_sector)(struct ums *ums_dev,
 			    ulong start, lbaint_t blkcnt, const void *buf);
-	void (*get_capacity)(struct ums *ums_dev,
-			     long long int *capacity);
+	unsigned int start_sector;
+	unsigned int num_sectors;
 	const char *name;
 	struct mmc *mmc;
-	int offset;
-	int part_size;
 };
 
 extern struct ums *ums;