Modify existing code, for automatically picking the spi bus number based
on Linux idr scheme as mentioned in FIXME.
This patch does the following:
(a) Remove the now unnecessary code which was allocating bus numbers using
    ATOMIC_INIT and atomic_dec_return macros.
(b) If we have an alias, pick the bus number from alias ID
(c) Convert to linux idr interface
Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>
Signed-off-by: Karthik Tummala <karthik@techveda.org>
Tested-by: Karthik Tummala <karthik@techveda.org>
Signed-off-by: Mark Brown <broonie@kernel.org>