power: pm8921-charger: make usb_max_current configurable

Allow the module parameter to be configurable through the board
files. This gives another option to set the usb_max_current
setting if userspace is not able to set this during
bootup.

Change-Id: I96886f8d0f3f03905163d93c4f8f3e637d98127f
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>