staging: vme: Add VME_BUS dependency to Kconfig

The KConfig file for VME_USER ('drivers/staging/vme/devices/Kconfig')
sourced at "drivers/vme/boards/KConfig" misses a `depends on` line for
VME_BUS, which is unnoticeable for menuconfig users who aren't be able
to select it through this interface without setting the CONFIG_VME_BUS
option because it's nested on VME_BUS menu entry.

A patch to move appropriately this driver to the "Staging" submenu
([PATCH 1/3], originally sent as a single patch submitted at
https://lore.kernel.org/linux-staging/3fbc5325e94b9ae0666a1f5a56a4e5372bfcea1d.camel@codeagain.dev/T/#t

),
however, unveiled this missing `depends on` line, since it moved the
`source` line for VME_USER's KConfig to somewhere else, allowing it to
be compiled without the support for VME_BUS. While it compiles fine and
seems to initialize the driver fine (tested both as module and as built-
in), it seems to make no sense having VME_USER without VME_BUS.

This patches addresses it by adding the VME_BUS to the `depends on` line
at "drivers/staging/vme/devices/Kconfig".

Signed-off-by: Bruno Moreira-Guedes <codeagain@codeagain.dev>
Link: https://lore.kernel.org/r/00de5644d7c2f8c8878eccf86b761e0602732089.1650544175.git.codeagain@codeagain.dev


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>