coresight: etm4x: Cleanup TRCRSCTLRn register accesses

This is a no-op change for style and consistency and has no effect on
the binary output by the compiler. In sysreg.h fields are defined as
the register name followed by the field name and then _MASK. This
allows for grepping for fields by name rather than using magic numbers.

Signed-off-by: James Clark <james.clark@arm.com>