mailbox: pcc: Drop handling invalid bit-width in {read,write}_register

pcc_chan_reg_init now checks if the register bit width is within the
list [8, 16, 32, 64] and flags error if that is not the case. Therefore
there is no need to handling invalid bit-width in both read_register
and write_register. We can drop that along with the return values for
these 2 functions.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>