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Follow convention of shorter names for smaller scope to fix
violations of MISRA rule 5.7

To prevent violation of directive 4.5 having variable name channel
in css_pm_scmi.c not being typographically ambiguous change macro
argument CHANNEL in css_mhu_doorbell.h change argument to _channel
to fit with our convention which is a permitted exception of
directive 4.5 for this project

Rule 5.7: A tag name shall be a unique identifier

Fixed for:
    make LOG_LEVEL=50 PLAT=juno

Change-Id: I147cdb13553e83ed7df19149b282706db115d612
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>