spi: Pick spi bus number from Linux idr or spi alias

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>