net: smc911x: Drop redundant CONFIG_SMC911X_16_BIT Kconfig symbol

The SMC911x Ethernet driver needs to know which accessor functions it
can use to access the MMIO registers. For that reason we have a Kconfig
choice between 16 and 32-bit bus width.

Since it's only those two options that we (and the Linux kernel)
support, and there does not seem to be any evidence of another bus
width anywhere, limit the Kconfig construct to a simple symbol.

This simplifies the code and allows a later rework to be much easier.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>