spi: omap: Allocate bus number from spi framework

spi framework should allocate bus number dynamically either
via Linux IDR or spi alias for master drivers. This patch deletes
code pertaining to manual allocation of spi bus number in spi omap2
master driver.

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>