Remove implicit enum conversions for clang
Remove implicit enum conversions for clang

gcc allows implicit conversion between enum type (even when `-Wall` is
specified).  clang does not allow such conversions with `-Wall`.  It
reports warnings like this one:

    error: implicit conversion from enumeration type 'os_error_t' (aka 'enum os_error') to different enumeration type 'ble_npl_error_t' (aka 'enum ble_npl_error')

This commit makes these conversions explicit via casts.  This allows the
unit test code to be built with clang using Mynewt's default build
flags.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble