SecurityPkg/RngDxe: Correctly update mAvailableAlgoArrayCount

mAvailableAlgoArrayCount holds the count of available RNG algorithms.
In a following patch, its value will be used to prevent the
EFI_RNG_PROTOCOL to be installed if no RNG algorithm is available.

Correctly set/reset the value for all implementations.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>