gas: sframe: improve code comments around SFRAME_FRE_BASE_REG_INVAL

Rename it to SFRAME_FRE_REG_INVALID.

gas/
        * gen-sframe.c (sframe_row_entry_new): Adjust code comments a
	bit.
        * gen-sframe.h (SFRAME_FRE_BASE_REG_INVAL): Rename from..
        (SFRAME_FRE_REG_INVALID): ..to this.