usb: ums: code refactoring to improve reusability on other boards.

This patch introduces some cleanups to ums code. Changes:

ums common:
- introduce UMS_START_SECTOR and UMS_NUM_SECTORS as defined in
  usb_mass_storage.h both default values as 0 if board config
  doesn't define them

common cleanup changes:
- change name of struct "ums_board_info" to "ums"
- "ums_device" fields are moved to struct ums and "dev_num" is removed
- change function name: board_ums_init to ums_init
- remove "extern" prefixes from usb_mass_storage.h

cmd_usb_mass_storage:
- change error() to printf() if need to print info message
- change return values to command_ret_t type at ums command code
- add command usage string

Changes v2:
ums common:
- always returns number of read/write sectors
- coding style clean-up
ums gadget:
- calculate amount of read/write from device returned value.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Marek Vasut <marek.vasut@gmail.com>