USB: gadget: f_mass_storage: drop START_TRANSFER() macro

This commit drops START_TRANSFER_OR() and START_TRANSFER()
macros with a pair of nice inline functions which are actually
more readable and easier to use.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>