net: ipa: define BCR values using an enum

The backward compatibility register (BCR) has a set of bit flags
that indicate ways in which the IPA hardware should operate in a
backward compatible way.  The register is not supported starting
with IPA v4.5, and where it is supported, defined bits all have the
same numeric value.

Redefine these flags using an enumerated type, with each member's
value representing the bit position that encodes it in the BCR.
This replaces all of the single-bit field masks previously defined.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>