stm32/spi: Replace all SPIx_I2S_BASE with SPIx_BASE

Latest versions of all reference manuals refer to the address as SPIx_BASE, and
simply name some of the individual registers as SPI_I2SXXXX.  Likewise, the
interrupts are simply SPIx, not SPIx/I2Sx.  Rather than hacking more duplicates
into the F0 and L0 parts where this was turning up, remove the pointless _I2S_
from SPI2/SPI3 and make it all consistent

Compile tested only, with the examples collection.

Signed-off-by: Karl Palsson <karlp@tweak.net.au>

Fixes #331
Fixes #347